1. Field of the Invention
This invention relates to fluid suspension of pulverized solids, and especially relates to magnetic separation of impurities from coal. It specifically relates to the removal of pyrite from coal by thermally enhancing the paramagnetism thereof and separating the pyrite by magnetic means.
2. REVIEW OF THE Prior Art
It is widely acknowledged that the United States is in the midst of a serious energy crisis and that coal must be much more intensively utilized in order to meet future energy requirements, if for no other reason than that coal reserves are far more abundant than reserves of all other non-nuclear fuels combined. However, burning of coal creates air and water pollution which has been the subject of considerable furor in recent years.
Sulfur content of coals used by public utilities for steam and electricity generation ranges from about 1 to 5 percent, so that during 1963 and in recent years, for example, about 5 million tons of sulfur were discharged into the atmosphere, mainly as sulfur dioxide. Sulfur occurs chiefly in three forms: (1) inorganic, (2) sulfate, and (3) organic. The inorganic sulfur is found as iron pyrite (FeS.sub.2 in isometric crystalline form), and marcasite (FeS.sub.2 in orthorhombic crystalline form), pyrite being more common and being found in coal as macroscopic and microscopic particles and as discrete grains, cavity fillings, fiber bundles, and aggregates.
Although the concentration of pyritic sulfur varies widely even within the same deposit, it normally varies from 0.2 to 3 percent on a sulfur basis. In coals containing more than 2 percent sulfur, about 1 percent is intimately tied up with the structure of the coal as organic sulfur and cannot be removed by mechanical means. Pyritic sulfur, however, can be removed by a variety of separation methods, including wet oil processing and dry methods such as air elutriation, electrostatic separation, and magnetic separation.
As noted by Trindade and Kolm in IEEE Transactions on Magnetics, Vol. Mag. 9, No. 3, September 1975, pyrite can be separated from coal in a water slurry flowing through a filamentary magnetic material packed into the bore of a solenoid magnet having a field of 20 kOe, particularly at slurry velocities less than 1 centimeter per second. It is recommended that the nature of the surfaces of the particles be chemically changed in order to generate areas of higher magnetic susceptibility. This advice was followed by Kindig et al, as disclosed in U.S. Pat. No. 3,938,966, by reacting coal particles with iron carbonyl at about 190 C.
The size distribution of pyrite particles in coals ranges from submicron to several millimeters. As disclosed by Ergun and Bean in Report of Investigations 7181 of the United States Bureau of Mines, the particle size of pyrite is logarithmetically equivalent to its weight percentage in a coal bed, each bed having its own characteristic relationship for pyrite particles. For example, on a weight basis, the Pittsburgh Number 8 bed in Ohio has an average particle size of about 50 microns, and the Mammoth bed in Iowa has an average particle size of about 110 microns. It is accordingly evident that coals must be finely pulverized in order to liberate such small particles of pyrite by any mechanical means.
Ergun and Bean further observed that coal particles have a magnetic susceptiblity of about -0.5 .times. 10-6 in cgs units and are consequently diamagnetic. Pyrite and many other mineral compounds are paramagnetic. In cgs units, pyrite has a magnetic susceptibility of 2800, and both gamma hematite and magnetite have a magnetic susceptibility of 15,600. Consequently, if less than 0.1 percent of pyrite in pyritic coal is converted to paramagnetic compounds of iron, the differential magnetic susceptibilities are sufficiently great that pyrite can be removed from powdered coal by magnetic means without recourse to a high-gradient magnetic field. Ergun et al confirmed that temperatures above the decomposition temperature of coal would be necessary in order to obtain sufficient conversion of pyrite to more magnetic forms and that decomposition reactions become detectable at temperatures well above 500.degree. C and have high energies of activation. They concluded that heating to temperatures above 600.degree. C for a few seconds would be sufficient.
It is known in the art to heat pulverized coal with a heated fluidizing gas and to maintain distillation and coking conditions, as disclosed in U.S. Pat. No. 2,608,526. Recycle gas is used according to U.S. Pat. No. 2,955,077 to fluidize pulverized agglomerative coals and, in a succession of fluidized stages, to dry and preheat the coal at 232.degree.-399.degree. C, to remove about 50% of the volatile matter at 385-441.degree. C for five minutes, and to remove tar vapors at 454.degree.-649.degree. C, using hot char at a weight ratio of 3:1 for heating the pulverized coal. A multi-stage process is also taught in U.S. Pat. No. 3,375,175 in which hot inert gas dries and preheats crushed coal in a fluidized bed at 316.degree.-343.degree. C to remove 0.5-5% oily liquid and water and raise the function temperature sufficiently for subsequent pyrolysis without agglomeration in 3 or more fluidized beds by passing a heated oxygen-containing gas countercurrently.
A process for producing fuel gas, sulfur, and char is additionally disclosed in U.S. Pat. No. 3,736,233 in which sensible heat is provided by inert gas or by char particles; desulfurization is achieved by passing pyrolyzed char, after treatment for up to 20 minutes at 1393.degree.-1343.degree. C, through a highintensity induced-roll magnetic separator. magnetic separation is also used in U.S. Pat. No. 3,463,310 after electromagnetic heating of coal particles to convert pyrite to pyrrhotite, magnetite, or hematite at temperatures on the order of 600.degree. C. A hydrogen-recycle process is discussed in U.S. Pat. No. 3,725,241 for hydrogenating coal under liquid phase conditions in a fluidized reaction zone at a temperature of 399.degree.-510.degree. C, magnetic separation being used at a field strength of about 1000 gauss.
Magnetic separators have long been proposed and used for magnetically separating two or more different substances having differing magnetic susceptibilities. For example, U.S. Pat. No. 689,561 teaches the downward passage of pulverized ores through the flared center of an electromagnet having a pair of opposed pole pieces. U.S. Pat. No. 1,729,008 describes an apparatus for impinging pulverized ores containing paramagnetic and diamagnetic contents onto the surface of a horizontally rotating drum having a stationary magnet therewithin.
What is need for large-scale electrical and steam generation, however, is not the conversion of coal into liquid fuels but the production of a rapidly burning fuel that is easily metered and has not zero sulfur content, with all organic sulfur removed, but a reasonably low content of sulfur, i.e. with most pyrites removed.
Particularly when burning bituminous, high bituminous, and sub-bituminous coals, in which the volatile matter is 35-50 percent by weight on a moisture-free basis, it is necessary to prevent agglomeration thereof while heating to a temperature high enough for enhancing the magnetic susceptibilities of its pyrite contents. It is further desirable to contain and pass along to the furnace combustion zone all evolved volatile matter in admixture with an adequate supply of oxygen for combustion. It is additionally desirable to be able to remove easily distillable oils for combustion purposes or for sale according to economic considerations.